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#card {
    width:124px; 
    height: 168px; 
    background-image: url('http://hosturl.co.uk/cc/cb.png');
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#card:hover {
    background-image:url('http://hosturl.co.uk/cc/cf.png');
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<div>
    <h1>I can guess the number you are thinking of</h1>
    <p>Follow the Rules and then hover the card below</p>
    <ol>
        <li>Think of a number below 10</li>
        <li>Double the number you have</li>
        <li>Add 6</li>
        <li>Divide it by 2</li>
        <li>Subtract the original number from your answer</li>
    </ol>
    <div id="card"></div>
</div>
